Image forming apparatus and image forming method

ABSTRACT

A gradation pattern is printed, the gradation pattern is corrected with a gamma correction table T O  based on a result in which the printed gradation pattern is read, the corrected gradation pattern is printed on a photosensitive drum or the like, and a measured density of the printed gradation pattern in S 4  is stored as a reference value of density D R . Thereafter, without user&#39;s operation, at a set automatic timing, the read gradation pattern P is corrected with the gamma correction table T O  and printed on the photosensitive drum or the like, density deviation ΔD between a measured density D of the printed gradation pattern in S 6  and the reference value of density D R  is determined, and the latest gamma correction table T is generated and updated on the basis of the density deviation ΔD. Consequently, high-quality image formation can be automatically performed.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based upon and claims the benefit of priorityfrom the prior Japanese Patent Application No. 2002-100483, filed Apr.2, 2002, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an image forming apparatus, inparticular relates to an image forming apparatus and an image formingmethod, in which a gamma correction table is updated with a simple way.

[0004] 2. Description of the Related Art

[0005] Recently various modes of image forming apparatus are developed,manufactured and widely spread as image information is treated asdigital information. In such an image forming apparatus, it is desirableto further increase image quality, a gamma correction table is providedin order to correspond to a change in characteristics of a scannerportion, a printer portion and the like, and the image information iscorrected with the gamma correction table to perform the image formationin order to correspond to the change in characteristics.

[0006] With reference to the image forming apparatus of the prior art,in generation of the gamma correction table, a gradation pattern storedin a storage area in the apparatus is printed, the printed gradationpattern is read by the scanner, and the gamma correction table isgenerated and updated on the basis of a result which has been read bythe scanner. Accordingly, a result of the desired and high-quality imageformation can be obtained with the correction in which physicalcharacteristics of the scanner portion, the printer portion and the likeare reflected at that time.

[0007] However, operation, in which a printed sheet of the gradationpattern having the built-in gradation pattern printed therein asdescribed above is moved by a user to a scanner table to be read withthe scanner, is at least generated in the update processing of the gammacorrection table in the apparatus of the prior art. In addition, theoperation for changing a normal operation mode to the gamma correctiontable update mode is also required. Thus, since the update processing ofthe gamma correction table in the apparatus of the prior art isburdensome for the user, and is not performed periodically andautomatically, when the user neglects the update processing of the gammacorrection table, there is a problem that it is not always possible toperform the high-quality image formation.

BRIEF SUMMARY OF THE INVENTION

[0008] It is an object of the invention to provide an image formingapparatus and an image forming method, in which high-quality imageformation applying easily an optimum gamma correction table can beperformed by automatically updating the gamma correction table in theimage forming apparatus.

[0009] In order to achieve the above object, according to an aspect ofthe present invention, there is provided an image forming apparatuscomprising: a pattern forming portion which reads gradation patterninformation stored in a predetermined storage area and forms an imageaccording to the gradation pattern information on an image recordingmedium; a first generating portion which reads the image according tothe gradation pattern information on the image recording medium formedby the pattern forming portion by using a scanner, and generates a firstgamma correction table on the basis of the read image information; afirst printing portion which reads the gradation pattern informationstored in the predetermined storage area, corrects the gradation patternaccording to the first gamma correction table generated by the firstgenerating portion, and prints an image according to the correctedgradation pattern information on an image carrier; a storage portionwhich detects a density of the image printed on the image carrier by thefirst printing portion, with a sensor for the amount of adhered toner,and stores the density of the image as a reference value of density in astorage area; a second printing portion which reads the gradationpattern information stored in the given storage area at an arbitrarytiming, corrects the gradation pattern information with the first gammacorrection table generated by the first generating portion, and printsan image according to the corrected gradation pattern information on theimage carrier; a calculating portion which detects the density of theimage printed on the image carrier by the second printing portion, withthe sensor for the amount of adhered toner, compares the density of theimage to the reference value of density stored in the storage portion,and calculates density deviation; a second generating portion whichgenerates a newly second gamma correction table on the basis of thedensity deviation which the calculating portion has calculated; and animage forming portion which reads image information on an original withthe scanner, corrects the read image information with the gammacorrection table formed by the second generating portion, and forms animage based on the corrected image information on the image recordingmedium.

[0010] As described above, the image forming apparatus of the inventiondecides the gamma correction table by printing and reading once thegradation pattern similarly to the apparatus of the prior art. Then, thegradation pattern is printed on an image carrier such as a drum anddensity is stored as reference density D_(R). However, thereafter, thegradation pattern is printed on the image carrier such as the drum andthe density D is detected in the same way at a predetermined arbitrarytiming, e.g., daily start-up of a copying machine or every three hoursin continuous use of the copying machine. The previously recordedreference density DR is compared to the detected density D to determinedensity deviation ΔD, and the gamma correction table is automaticallyupdated according to the density deviation ΔD. Consequently, the usercan automatically update the optimum gamma correction table at the settiming without performing any special operation.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0011]FIG. 1 is a flow chart schematically showing an example ofautomatic update processing of a gamma correction table according to theinvention;

[0012]FIG. 2 is a sectional view showing an example of a structure of animage forming apparatus according to the invention;

[0013]FIG. 3 is a block diagram showing an example of a configuration ofthe image forming apparatus according to the invention;

[0014]FIGS. 4A and 4B are flow charts illustrating in detail anembodiment of the automatic update processing of the gamma correctiontable according to the invention;

[0015]FIGS. 5A and 5B are flow charts illustrating in detail an exampleof the automatic update processing of the gamma correction tableaccording to the invention;

[0016]FIG. 6 is a flow chart illustrating in detail an example of theautomatic update processing of the gamma correction table according tothe invention;

[0017]FIG. 7 is a graph for illustrating definition of each axis of thegraph illustrating the automatic update processing of the gammacorrection table according to the invention;

[0018]FIG. 8 is a graph showing image density of a sample in which agradation sheet for gamma correction is printed by a printer regardingthe automatic update processing of the gamma correction table accordingto the invention;

[0019]FIG. 9 is a graph showing reflectivity regarding the automaticupdate processing of the gamma correction table according to theinvention when the gradation sheet for gamma correction is read by ascanner;

[0020]FIG. 10 is a graph showing a case in which the reflectivity istransformed into an area ratio regarding the automatic update processingof the gamma correction table according to the invention;

[0021]FIG. 11 is a graph showing calculation of a gamma correction curve(a gradation number L to the area ratio Z) regarding the automaticupdate processing of the gamma correction table according to theinvention;

[0022]FIG. 12 is a graph showing relationship regarding the automaticupdate processing of the gamma correction table according to theinvention when data of the area ratio Z is gamma-corrected to betransformed into print/scanning/area ratio;

[0023]FIG. 13 is a graph showing an example of fluctuation in printergradation characteristics regarding the automatic update processing ofthe gamma correction table according to the invention;

[0024]FIG. 14 is a graph showing reference densities d1 and d2 measuredby a sensor regarding the automatic update processing of the gammacorrection table according to the invention;

[0025]FIG. 15 is a graph showing sensor densities d1′ and d2′ during thefluctuation in the density regarding the automatic update processing ofthe gamma correction table according to the invention;

[0026]FIG. 16 is a graph showing relationship between density deviationand the reflectivity regarding the automatic update processing of thegamma correction table according to the invention;

[0027]FIG. 17 is a graph showing approximation calculated by the sensordensity deviation regarding the automatic update processing of the gammacorrection table according to the invention;

[0028]FIG. 18 is a graph showing a process of generating correction LUTfrom the sensor density deviation AD regarding the automatic updateprocessing of the gamma correction table according to the invention; and

[0029]FIG. 19 is a graph showing an image density after correctionregarding the automatic update processing of the gamma correction tableaccording to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0030] An embodiment of an image forming apparatus according to theinvention will be described below referring to the drawings. Though theinvention is characterized by the characteristic processing whichautomatically updates a gamma correction table in the image formingapparatus, at first a structure of the embodiment of the image formingapparatus to which the processing of the invention is applied will bedescribed in detail below by using the drawings.

[0031] [Example of Structure of Image Forming Apparatus According to theInvention]

[0032]FIG. 2 is a sectional view showing an example of the structure ofthe image forming apparatus according to the invention, and FIG. 3 is ablock diagram showing an example of a configuration of the image formingapparatus according to the invention.

[0033] As shown in FIG. 2, a color laser printer which is the imageforming apparatus is divided mainly into an optical system 13 includinga scanner and an image forming portion F. The image forming portion Fhas a photosensitive drum 1 as an image carrier, and the photosensitivedrum 1 is provided so as to be rotatable in a counterclockwise directionto the paper. Further, the image forming portion F includes a chargingdevice 2 which is a charging portion and arranged near thephotosensitive drum 1, a first developing device 4, a second developingdevice 5, a third developing device 6, and a fourth developing device 7,which are a developing portion, a sensor 8 for the amount of adheredtoner which detects toner density, a transfer drum 9 which is a transfermaterial support, a static eliminator 10 before cleaning, a cleaner 11,and an antistatic lamp 12.

[0034] The image carrier means a portion bearing the image, and includesnot only the photosensitive drum 1 but also the transfer drum 9. In theimage of the gradation pattern which is formed in the image carrierduring a step of generating a gamma correction table according to theinvention, its subject includes the photosensitive drum 1 and thetransfer drum 9.

[0035] A surface of the photosensitive drum 1 is charged by the chargingdevice 2. A laser beam 14 emitted from a laser exposing device 13serving as a scanner portion and an exposure portion exposes the surfaceof the photosensitive drum 1 between the charging device 2 and the firstdeveloping device 4, and thereby an electrostatic latent image is formedaccording to image data.

[0036] The first to fourth developing devices 4 to 7 develop theelectrostatic latent images on the photosensitive drum 1 correspondingto each color to a color toner image, for example, the first developingdevice 4 develops magenta, the second developing device 5 develops cyan,the third developing device 6 develops yellow, and the fourth developingdevice 7 develops black.

[0037] On the other hand, a transfer paper of the transfer material issent away from a paper feeding cassette 15 by a paper feeding roller 16,adjusted temporarily by a resist roller 17, sent so as to be absorbed ata predetermined position on the transfer drum 9 by the resist roller 17,and electrostaticically adsorbed onto the transfer drum 9 by a adsorbingroller 18 and a adsorption charging device 19. The transfer paper iscarried with the clockwise rotation of the transfer drum 9, while thetransfer paper is adsorbed on the transfer drum 9.

[0038] The developed toner image on the photosensitive drum 1 istransferred on the transfer paper with a transfer charging device 20 ata position where the photosensitive drum 1 opposes to the transfer drum9. In the case of color print, the process in which one period is onerotation of the transfer drum 9 is performed in plural times while thedeveloping device is changed, and the toner image having the pluralcolors is multiple-transferred on the transfer paper.

[0039] The transfer paper having the toner image transferred thereto isfurther carried with the rotation of the transfer drum 9, and the staticin the transfer paper is eliminated with an inner static eliminator 21before separation, an outer static eliminator 22 before separation, anda separating static eliminator 23. Then, the transfer paper is peeledfrom the transfer drum 9 with a separating claw 24 and carried to afixing device 27 with carrying belts 25 and 26. The toner on thetransfer paper, which has been heated with the fixing device 27, ismelted and fixed to the transfer paper immediately after the transferpaper is ejected from the fixing device 27. The transfer paper in whichthe fixing has been completed is ejected to a tray 28.

[0040] As shown in FIG. 3, in the above-described color laser printer,the charging device 2 mainly includes a charging wire 31, a conductivecase 32, and a grid electrode 33. The charging wire 31 is connected to ahigh-voltage power supply 34 for corona, and the charging wire 31charges the surface of the photosensitive drum 1 by corona discharge.The grid electrode 33 is connected to a high-voltage power supply 35 forgrid bias, and the grid bias voltage controls the amount of charge tothe surface of the photosensitive drum 1.

[0041] The surface of the photosensitive drum 1, which has beenuniformly charged with the charging device 2, is exposed by themodulated laser beam 14 from the laser exposing device 13, and theelectrostatic latent image is formed. A gradation data buffer 36 storesgradation data from an external device or a controller, which is notshown, corrects gradation characteristics of the printer, and transformsthe gradation characteristics into laser exposing time (pulse width)data.

[0042] A laser driving circuit 37 modulates laser-driving current(emission time) according to the laser exposing time data from thegradation data buffer 36 so as to synchronize a scanning position of thelaser beam 14. The modulated laser-driving current drives asemiconductor laser oscillator (not shown) in the laser exposing device13. Accordingly, the semiconductor laser oscillator performs emittingoperation according to the exposing time data.

[0043] Further, the laser driving circuit 37 compares an output to a setvalue of a light receiving element for monitoring (not shown) in thelaser exposing device 13, and performs control in which the drivingcurrent maintains an output light quantity of the semiconductor laseroscillator at a set value.

[0044] A pattern generating circuit 38 stores image information of agradation correcting sheet in order to output the gradation correctingsheet for the automatic update processing of the gamma correction table.

[0045] The developing device 4 develops the electrostatic latent imageformed on the surface of the photosensitive drum 1. That is, thedeveloping device 4 is, for example, a two-component developing method,and stores a developer including the toner and a carrier. A weight ratioof the toner in the developer (hereinafter referred to as tonerconcentration) is measured by a toner concentration measuring portion39. A toner supplying motor 41 which drives a toner supplying roller 40is controlled according to an output of the toner concentrationmeasuring portion 39, the toner in a toner hopper 42 is supplied intothe developing device 4.

[0046] A developing roller 43 of the developing device 4 is formed by aconductive member and connected to a high-voltage power supply 44 forapplying developing bias. The developing roller 43 rotates with thedeveloping bias voltage applied, and performs development in such amanner that the toner adheres to the electrostatic latent image on thephotosensitive drum 1. The toner image on the photosensitive drum 1,which has been developed in the above-described manner, is transferredto the transfer paper by the transfer drum 9.

[0047] A control circuit 45 causes the gradation data to generate fromthe pattern generating circuit 38 and exposes the gradation pattern onthe photosensitive drum 1, when warm-up processing is terminated afterturning on the power.

[0048] When the gradation pattern comes to a position where thegradation pattern opposes to the sensor 8 for the amount of adheredtoner, the sensor 8 for the amount of adhered toner measures the tonerconcentration of the gradation pattern. The output of the sensor 8 forthe amount of adhered toner is digitized by an A/D converter 46 andinputted to the control circuit 45.

[0049] AS described later, the control circuit 45 performs the updateprocessing of the gamma correction table, which uses the gradationpattern.

[0050] The high-voltage power supplies 35 and 44 are controlled by anoutput voltage control signal which is supplied from the control circuit45 through D/A converters 47 and 48, respectively.

[0051] A rewritable storage portion 61 including EEPROM and the like, inwhich data is not erased even if the power is turned off, a storageportion 62 including RAM and the like for data storage, a timer 63 whichmeasures stand-by time, and a CPU 64 which controls the whole thecontrol circuit 45 are provided in the control circuit 45.

[0052] In the storage portion 61, various kinds of set values arepreviously stored such as initial grid bias voltage value anddevelopment bias voltage value which corresponds to bias conditions asreference gradation characteristics at room temperature and roomhumidity, gradation data of test pattern, a coefficient indicatingsurface potential characteristics, a predetermined paper count ofprinting, a predetermined time interval, the maximum number of controls,values of bias conditions, an abnormal range of the sensor 8 for theamount of adhered toner, and quantity of reflected light.

[0053] With reference to the values of bias conditions, there are thegrid bias, each upper limit value and lower limit value (predeterminedrange) of development biases, and a permissible range of a voltagedifference between the grid bias and the development bias. In a targetvalue of the above-described test pattern portion, the change in aninput and display can be performed with a control panel 49.

[0054] In the storage portion 62 which has a function of a primarystorage portion or a temporarily storage portion, while the bias valuewhich has been set before the sensor 8 for the amount of adhered tonersenses the abnormal event is stored (the bias value is stored when abias changing mode is set), a counter which counts the number ofcontrols, a counter which counts the paper count of printing, a sensorabnormal flag which is flagged when the sensor 8 for the amount ofadhered toner senses the abnormal event, and a toner empty flag which isflagged when the toner is empty are provided.

[0055] Further, information such as a gamma correction table T_(O)according to the invention, a gamma correction table T to be updated,and a reference value of density D_(R), which are described later, isstored in the storage portion 61 as appropriate.

[0056] [Automatic Update Processing of Gamma Correction Table Accordingto the Invention]

[0057] The automatic update processing of the gamma correction tableaccording to the invention in the above-described color laser printer asthe image forming apparatus will be described in detail below by usingthe flow chart.

[0058] (Schematic Description)

[0059]FIG. 1 is a flow chart schematically showing an example of theautomatic update processing of the gamma correction table according tothe invention.

[0060] In the flow chart, initialization processing, in which theinitial gamma correction table T_(O) and the reference value of densityD_(R) in the steps S1 to S5 are determined, is performed as aprecondition for the automatic update processing of the gamma correctiontable. Then, the automatic update processing of the gamma correctiontable is performed at an arbitrary timing on the basis of these valuesof the initial set processing.

[0061] For the first initialization processing, a gradation pattern P isread from the pattern generating circuit 38 and the gradation pattern isprinted by the image forming portion F (S1). The sheet of the printedgradation pattern P is set on an original platen (not shown) as a useroperation and read with the scanner portion 13 (S2). The gammacorrection table T_(O) is generated according to the read imageinformation (S3). The image information of the gradation pattern P isread from the pattern generating circuit 38 again, and the image isprinted on the photosensitive drum 1 (or transfer drum 9) which is theimage carrier according to the image information which has beencorrected by the gamma correction table T_(O) generated in the step S3(S4). The gradation pattern P printed on the image carrier is measuredwith the sensor 8 for the amount of adhered toner, and the measureddensity is stored in, e.g., the storage portion 61, setting the measureddensity to the reference value of density D_(R) (S5).

[0062] Then, for the automatic update processing of the gamma correctiontable, the gradation pattern P is read from the pattern generatingcircuit 38 at an arbitrary timing and the image is printed on the imagecarrier according to the image information which has been corrected withthe gamma correction table T_(O) generated in the step S3 (S6). Theprinted gradation pattern P is measured with the sensor 8 for the amountof adhered toner, and a density deviation ΔD is determined, comparingthe measured value of density D to the stored reference value of densityD_(R) (S7). Finally the new gamma correction table T is determined onthe basis of the determined density deviation ΔD (S8). Various kinds ofmethods of calculating the new gamma correction table T from the densitydeviation ΔD are thought and it is not limited to one method, however,one of the methods is described later by using the drawings.

[0063] The operation, in which the sheet of the gradation pattern storedinside is printed and read in each time, is required in the conventionalapparatus, when the gamma correction table T is updated according toenvironmental change such as temperature change and secular change,however, simplification is achieved as follows in the invention.

[0064] That is, similarly to the conventional apparatus, the gammacorrection table is determined by printing and reading the gradationpattern once. Then, the gradation pattern is printed on the imagecarrier such as the drum and the density is stored as the referencedensity D_(R). However, thereafter, the gradation pattern is similarlyprinted on the image carrier such as the drum and the density D isdetected at an arbitrary timing set by a user and the like, i.e., at atiming selected by the user such as every morning or every three hours.Comparing the recorded reference density D_(R) to the detected densityD, the density deviation ΔD is calculated, and the gamma correctiontable is automatically updated according to the density deviation ΔD.Such processing allows the user to automatically update the optimumgamma correction table for present conditions of the devices at anarbitrary timing without performing any special operation.

[0065] (Detail Description)

[0066]FIGS. 4A, 4B, 5A, 5B and 6 are flow charts illustrating in detailan example of the automatic update processing of the gamma correctiontable according to the invention. FIGS. 4A and 4B show initializationprocessing, and FIGS. 5A, 5B and 6 show automatic update processing.

[0067] FIGS. 7 to 19 are graphs regarding the automatic updateprocessing of the gamma correction table according to the invention.FIGS. 8 to 14 are graphs illustrating the initialization processing forthe automatic update processing, and FIGS. 15 to 19 are graphsillustrating the automatic update processing itself.

1. Initialization Processing

[0068] The initialization processing will be described by using the flowchart in FIGS. 4A and 4B.

[0069] First the graph illustrating a process of determining the gammacorrection table is described. FIG. 7 is the graph for illustratingdefinition of each axis of the graph illustrating the automatic updateprocessing of the gamma correction table according to the invention. Thefirst quadrant is the graph showing the relation between the gradationnumber L and image density ID in the printer, the second quadrant is thegraph showing the relation between reflectivity R and the image densityID in the scanner, the third quadrant is the graph showing the relationbetween the reflectivity R and the area ratio Z in color transformationprocessing, and the fourth quadrant is the graph showing the relationbetween an area ratio Z and the gradation number L in the gammacorrection. The gamma correction table T is obtained by following therelations from the first quadrant to the fourth quadrant.

[0070] In the flow charts of FIGS. 4A and 4B, the initializationprocessing is described in detail as the concrete processing. Theprocessing is performed by control of the CPU 64 shown in FIG. 3, andthe processing is classified into three parts according to contents ofthe processing in the flow charts of FIGS. 4A and 4B, i.e., theprocessing of SYS-CPU which is the systematic processing, the processingof LGC-CPU which is the mechanical operation processing, and theprocessing of IMC-CPU which is the image information processing.However, these three parts of the processing may be performed with oneCPU or three CPUs and three programs. Accordingly, the CPU 63 may be oneCPU or a generic term of the plural CPUS.

[0071] The gradation pattern P is read from the pattern generatingcircuit 38, and a predetermined gradation pattern P is printed (S11).The sheet of the printed gradation pattern P is set on the originalplaten (not shown) as the user operation and scanned by the scannerportion 13 (S12). Then, the gamma correction table T_(O) is generatedaccording to the read image information (S13).

[0072] The graphs in FIGS. 8 to 11 illustrate the process until thegamma correction table T_(O) is calculated. That is, FIG. 8 is the graphshowing the image density of a sample in which the gradation sheet forthe gamma correction is printed by the printer, FIG. 9 is the graphshowing the reflectivity when the gradation sheet for the gammacorrection is scanned by the scanner, FIG. 10 is the graph showing thecase in which the reflectivity is transformed into the area ratio, FIG.11 is the graph showing the calculation of the gamma correction curve(the gradation number L to the area ratio Z), and FIG. 12 is the graphshowing the relationship when the area ratio Z data is transformed intoprint/scan/area ratios with the gamma correction. In FIG. 12, areference look-up table (LUT) of the gamma correction table isgenerated.

[0073] When a request command for start of acquiring reference densityis given to the CPU 63 from the state of the step S13, an valid decisionwhether or not acquiring operation of the sensor reference density isperformed is started by receiving the request command. Further, statusin execution is outputted from LGC-CPU and the status in execution isreceived with SYS-CPU, so that operating state of the image formingapparatus can be confirmed by the user, for example, “EXECUTION OFCALIBRATION” can be displayed on the control panel.

[0074] A setting-register value of related matter of test pattern issaved according to the valid decision (S14). Then, the decision of thevalidity or invalidity is performed (S15). The validity or invalidity isformed so as to be changed by a service person or the like, theprocessing is terminated without executing the acquiring operation ofthe reference density (or measured density) when the decision is theinvalidity.

[0075] When the decision is the validity, the initial processing isperformed (S16). Then, a request status of start of print sequence isgiven to LGC-CPU. This status is a trigger starting operation of theimage forming process. The status is one which starts rotating operationof a motor or the like for driving the photosensitive drum and thetransfer drum, and turns on various kinds of high-voltage power suppliessuch as the charging/development to allow the apparatus to form theimage.

[0076] When the request status of start of print sequence is given toLGC-CPU, the print sequence is started (S17). That is, each motor is setin motion and the process operation is started. When a transmissioncommand for print permission, i.e., the command notifying that theapparatus has become the state in which the image can be formed is givento IMC-CPU, the calibration of the sensor 8 for the amount of adheredtoner is performed and a calibration result status, i.e., the statustransmitting the calibration result of the sensor is outputted to theLGC-CPU (S18).

[0077] Next the setting-register value of related matter of test patternis saved, i.e., various kinds of setting (a start position or finishposition of the image, selection of the gradation data, and the like)are performed during the generation of the test pattern, the originalsetting is saved (stored) such that a common register which is used evenin other image forming operation can be returned to the original setting(S19).

[0078] According to the saving of the setting-register value of relatedmatter of test pattern, a request status of the test pattern, i.e., thestatus notifying generating color, a kind, the gradation number, and thelike of the test pattern is outputted and the test pattern is set (S20).The various kinds of setting are finished on the basis of a startcommand for rendering of test pattern, i.e., the notification in therequest status of test pattern, and the register value of related matterof test pattern is set when the command notifying that the test patterncan be rendered is outputted (S21).

[0079] According to the setting of the register value, VSYNC attentionwhich is a signal for taking start timing to write the image becausesampling timing of the test pattern is controlled with a sensor inIMC-CPU, i.e., a synchronizing signal of sub-scanning (VSYNC) isgenerated. An valid signal of sub-scanning width (VDEN) is outputted byreceiving the synchronizing signal of sub-scanning (VSYNC) (S22), theimage data of the sub-scanning width of the test pattern is outputted(S23). The sampling of the sensor for the amount of adhered toner isperformed (S24).

[0080] According to the sampling, a transmission status of patterndetecting result is outputted, i.e., the status notifying the sensoroutput result in the reading of the test pattern is outputted (S25).

[0081] According to the sensor output, a transmission status of densityvalue is outputted, i.e., the density value is determined on the basisof the calibration result of the sensor and the sensor output resultduring the reading of the test pattern, and the status for notifying thedensity value is outputted. The determined value of the referencedensity D_(R) is recorded in the storage portion 61 or the like (S26).

[0082] At this point, decision of pattern termination is performed(S27). That is, when the pattern to be generated is across plurality(the pattern is generated by the many gradation numbers), sometimesthere is the case in which the rendering is not completed in one cycle.In this case, the sampling operation is performed by dividing the manygradation numbers into some cycles. In the case of the plural cycles,whether or not the sampling of the whole pattern is terminated (all thecycles are terminated) is decided at this decision part.

[0083] When the sampling of the whole pattern is normally terminated, aresult status of acquiring reference density, i.e., the status notifyingthat the sampling of the whole test pattern is normally terminated inthe acquisition of the reference density is outputted, and theacquisition of the reference value of density D_(R) is terminated (S28).

[0084]FIG. 13 is the graph showing an example of the deviation ofprinter gradation characteristics, and FIG. 14 is the graph showingmeasurement of the reference densities dl and d2 by the sensor. FIGS. 13and 14 illustrate the process in which the reference value of densityD_(R) is determined on the basis of the reference look-up tablegenerated in the step S13.

[0085] Thus, the gamma correction table T_(O) as an initial value andthe reference value of density D_(R) are determined by theabove-described initialization processing.

2. Automatic Update Processing

[0086] The automatic update processing will be described below by theflow charts in FIGS. 5A, 5B and 6. The automatic update processing isone in which the measured value of density D of the gradation patternformed on the image carrier is detected at the arbitrary timing andcompared to the reference value of density D_(R) measured in theinitialization processing to determine the density deviation ΔD and thelatest gamma correction table T is generated on the basis of the densitydeviation ΔD.

[0087]FIG. 15 is the graph showing the sensor densities d1′ and d2′during the deviation in the density, FIG. 16 is the graph showing therelationship between the density deviation and the reflectivity, FIG. 17is the graph showing approximation calculated by the sensor densitydeviation, FIG. 18 is the graph showing the process of generating thecorrection LUT from the sensor density deviation ΔD, and FIG. 19 is thegraph showing the image density after the correction.

[0088] In the flow charts of FIGS. 5A and 5B, when a request command forstart of correction control is received, the setting-register value ofrelated matter of test pattern is saved (S31). The user can freely setthe timing when the request command for start of correction control issubmitted, or the timing is appropriately factory-adjusted. For example,when the apparatus is started in motion, the automatic update of thegamma correction table may be performed one a day or once every severalhours, or the automatic update may be commanded from the control panel49. Further, the setting of a date and time and the setup of theautomatic update of the gamma correction table may be performed bycommunication.

[0089] The decision of validity or invalidity is performed according tothe request command for start of correction control (S32). The validityor invalidity is formed so as to be changed by the service person or thelike. When the decision is the invalidity, the processing is terminatedwithout executing the acquiring operation of the measured value ofdensity.

[0090] When the decision is the validity, the initial processing isperformed (S33). Sequentially, the request status of start of printsequence is given. This status is a trigger starting the operation ofthe image forming process. The status is one which starts the rotatingoperation of the motor or the like for driving the photosensitive drumand the transfer drum, and turns on various kinds of high-voltage powersupplies such as the charging/development to allow the apparatus to formthe image.

[0091] When the request status of start of print sequence is given toLGC-CPU, the print sequence is started (S34). That is, each motor is setin motion and the process operation is started.

[0092] Next, the register setting value of related matter of testpattern is saved, i.e., various kinds of setting (the start position orfinish position of the image, selection of the gradation data, and thelike) are performed during the generation of the test pattern, and theoriginal setting is saved (stored) so that the common register which isused even in other image forming operation can be returned to theoriginal setting (S35).

[0093] According to the saving of the setting-register value of relatedmatter of test pattern, the request status of test pattern, i.e., thestatus notifying generating color, a kind, the gradation number, and thelike of the test pattern is outputted and the test pattern is set (S36).The various kinds of setting are finished on the basis of the startcommand for rendering of test pattern, i.e., the notification in therequest status of test pattern, and the register value of related matterof test pattern is set when the command notifying that the test patterncan be rendered is outputted (S37).

[0094] According to the setting of the register value, VSYNC attentionwhich is the signal for taking the start timing to write the imagebecause the sampling timing of the test pattern is controlled with thesensor in IMC-CPU, i.e., the synchronizing signal of the sub-scanning(VSYNC) is generated. The valid signal of the sub-scanning width (VDEN)is outputted by receiving the synchronizing signal of the sub-scanning(VSYNC) (S38), the image data of the sub-scanning width of the testpattern is outputted (S39). The sampling of the sensor for the amount ofadhered toner is performed (S40).

[0095] According to the sampling, the transmission status of patterndetecting result is outputted, i.e., the status notifying the sensoroutput result in the reading of the test pattern is outputted (S41).According to the sensor output, the transmission status of density valueis outputted, i.e., the density value is determined on the basis of thecalibration result of the sensor and the sensor output result during thereading of the test pattern, and the status for notifying the densityvalue is outputted. The determined value of the density D is recorded inthe storage portion 61 or the like (S42).

[0096] At this point, decision of pattern termination is performed(S43). That is, when the pattern to be generated is across plurality(the pattern is generated by the many gradation numbers), sometimesthere is the case in which the rendering is not completed in one cycle.In this case, the sampling operation is performed by dividing the manygradation numbers into some cycles. In the case of the plural cycles,whether or not the sampling of the whole pattern is terminated (all thecycles are terminated) is decided at this decision part.

[0097] Then, comparing the measured value of density D which is measuredin the step S42 to the reference value of density D_(R) which ismeasured in the step S26, the density deviation ΔD is determined (S44).The decision of approval/disapproval is performed, i.e., the deviationbetween the measured value of density D and the reference value ofdensity D_(R) is calculated, and whether or not the deviation ΔD iswithin the predetermined permissive range is decided. When the deviationΔD is within the predetermined permissive range, assuming that theresult is normal, the gamma correction table is not changed and the newtable is not generated (S45).

[0098] When the deviation ΔD is out of the predetermined permissiverange, the gamma correction table is changed and processing of themaximum number of correction is performed (S46). That is, the maximumnumber of correction is an upper limit number by which the gammacorrection table can be newly generated. When the processing has reachedthe maximum number of correction, even if the result of the decision ofapproval/disapproval is disapproval, the processing is terminatedwithout generating the new gamma correction table.

[0099] When the processing has not reached the maximum number ofcorrection, the gamma correction table is updated and the request statusof updating gamma correction table is outputted, i.e., the status whichrequires the update of the gamma correction table is outputted when thenew gamma correction table is generated by the decision ofapproval/disapproval and the processing of the maximum number ofcorrection. This status causes the density deviation ΔD to be determinedby comparing the measured value of density D to the reference value ofdensity D_(R), and a kind, object color, and the like of the newlygenerating gamma correction table is notified (S47).

[0100] Then, the density deviation ΔD for the reflectivity R ispredicted (S48). A ratio of reflectivity deviation k is determined ask[L]=10^(−ΔD) from the density deviation ΔD, estimation of deviationreflectivity R′ is determined as R′=kR on the basis of the ratio ofreflectivity deviation k, and the amount of the deviation of the arearatio Z is converted (S49). A fluctuating gamma curve is predictedaccording to the conversion (S50). The new gamma correction table T iscalculated according to the prediction of the fluctuating gamma curve(S51). Finally, the new gamma correction table T is stored as executingdata in the storage portion 61 or the like.

[0101] After the storage is completed, a posting command for completionof correction processing is outputted, i.e., the status notifying thatthe gamma correction table is newly generated on the basis of thedeviation between the sensor reference value and the measured value andits storage is terminated is outputted.

[0102] According to the posting command, the decision whether or notfeedback is valid or invalid is performed, i.e., the decision whetherthe test pattern is generated again by the newly generated valid gammacorrection table to perform feedback control is performed by a feedbackdeciding flag (S53).

[0103] In the case that the feedback deciding flag is invalid, even ifthe gamma correction table is newly generated, the generation of thetest pattern and the sampling of the sensor output are not performedwith the valid gamma correction table (it becomes open loop control).Then, a result status of correction control, which shows the normaltermination of the correction control, is transmitted.

[0104] By using the process described above in detail, the gammacorrection table is automatically updated to the optimal value at thearbitrary timing, unlike the user prints the gradation pattern and makesthe scanner read the printed gradation pattern in the prior art.Consequently, even if the scanner and printer portion are changed by thetemperature change and the secular change, it is possible to provide theimage forming apparatus, in which the user can automatically perform thehigh-quality image formation according to the change in characteristicsof the apparatus without re-setting the gamma correction table.

[0105] However, those skilled in the art can realized the invention bythe various kinds of embodiments described above, it is easy for thoseskilled in the art to further conceive various kinds of variations ofthe embodiments, and it is possible without inventive ability to applythe invention to various modes. Accordingly, the invention extends thewide range which is not contradictory to a disclosed principle and a newfeature, and it is not limited to the above-described embodiment.

[0106] As described above, according to the invention, the referencevalue of density D_(R) is compared to the measured value of density D todetermine the density deviation ΔD, and the gamma correction table isautomatically updated according to the density deviation ΔD at thepredetermined arbitrary timing, e.g., the daily start-up of the copyingmachine or every three hours in continuous use of the copying machine.Consequently, since the user can automatically update the optimum gammacorrection table at the predetermined timing without performing anyspecial operation, it is possible to provide the image forming apparatusin which the high-quality image can be automatically formed.

What is claimed is:
 1. An image forming apparatus comprising: a patternforming portion which reads gradation pattern information stored in apredetermined storage area and forms an image according to the gradationpattern information on an image recording medium; a first generatingportion which reads the image according to the gradation patterninformation on the image recording medium formed by the pattern formingportion by using a scanner, and generates a first gamma correction tableon the basis of the read image information; a first printing portionwhich reads the gradation pattern information stored in thepredetermined storage area, corrects the gradation pattern according tothe first gamma correction table generated by the first generatingportion, and prints an image according to the corrected gradationpattern information on an image carrier; a storage portion which detectsa density of the image printed on the image carrier by the firstprinting portion, with a sensor for the amount of adhered toner, andstores the density of the image as a reference value of density in astorage area; a second printing portion which reads the gradationpattern information stored in the given storage area at an arbitrarytiming, corrects the gradation pattern information with the first gammacorrection table generated by the first generating portion, and printsan image according to the corrected gradation pattern information on theimage carrier; a calculating portion which detects the density of theimage printed on the image carrier by the second printing portion, withthe sensor for the amount of adhered toner, compares the density of theimage to the reference value of density stored in the storage portion,and calculates density deviation; a second generating portion whichgenerates a newly second gamma correction table on the basis of thedensity deviation which said calculating portion has calculated; and animage forming portion which reads image information on an original withthe scanner, corrects the read image information with the gammacorrection table formed by the second generating portion, and forms animage based on the corrected image information on the image recordingmedium.
 2. An image forming apparatus according to claim 1, wherein theimage carrier on which the first and second printing portions print theimage is a photosensitive drum.
 3. An image forming apparatus accordingto claim 1, wherein the image carrier on which the first and secondprinting portions print the image is a transfer drum.
 4. An imageforming apparatus according to claim 1, wherein the second generatingportion performs interpolation processing according to a plurality ofgradation numbers for the density deviation determined by thecalculating portion, determines a ratio of reflectivity deviation fromthe density deviation in which the interpolation processing isperformed, determines a conversion curve of scanner reflectivity fromthe ratio of reflectivity deviation, and generates a newly second gammacorrection table from the conversion curve of scanner reflectivity. 5.An image forming apparatus according to claim 1, wherein each step ofthe second printing portion, the calculating portion, and the secondgenerating portion is automatically performed at a predeterminedinterval without user's instruction, and thereby the second gammacorrection table is automatically updated.
 6. An image forming methodcomprising: a pattern forming step of reading gradation patterninformation stored in a predetermined storage area and forming an imageaccording to the gradation pattern information on an image recordingmedium; a first generating step of reading a gradation pattern on theimage recording medium formed in said pattern forming step by using ascanner, and generating a first gamma correction table on the basis ofthe read image information; a first printing step of reading thegradation pattern information stored in the predetermined storage area,correcting the gradation pattern according to the first gamma correctiontable generated in the first generating step, and printing an imageaccording to the corrected gradation pattern information on an imagecarrier; a storing step of detecting a density of the image printed onthe image carrier in the first printing step, with a sensor for theamount of adhered toner, and storing the density of the image as areference value of density in a storage area; a second printing step ofreading the gradation pattern information stored in the predeterminedstorage area at an arbitrary timing, correcting the gradation patterninformation with the first gamma correction table generated in the firstgenerating step, and printing an image according to the correctedgradation pattern information on the image carrier; a calculating stepof detecting the density of the image printed on the image carrier inthe second printing step, with the sensor for the amount of adheredtoner, comparing the density of the image to the reference value ofdensity stored in the storage step, and calculating density deviation; asecond generating step of generating a newly second gamma correctiontable on the basis of the density deviation calculated in thecalculating step; and an image forming step of reading image informationon an original with the scanner, correcting the read image informationwith the gamma correction table formed in the second generating step,and forming an image based on the corrected image information on theimage recording medium.
 7. An image forming method according to claim 6,wherein the image carrier on which the image is printed in the first andsecond printing steps is a photosensitive drum.
 8. An image formingmethod according to claim 6, wherein the image carrier on which theimage is printed in the first and second printing steps is a transferdrum.
 9. An image forming method according to claim 6, wherein thesecond generating step performs interpolation processing according to aplurality of gradation numbers for the density deviation determined inthe calculating step, determining a ratio of reflectivity deviation fromthe density deviation in which the interpolation processing isperformed, determines a conversion curve of scanner reflectivity fromthe ratio of reflectivity deviation, and generates a newly second gammacorrection table from the conversion curve of scanner reflectivity. 10.An image forming method according to claim 6, wherein each step of thesecond printing step, the calculating step, and the second generatingstep is automatically performed at a predetermined interval withoutuser's instruction, and thereby the second gamma correction table isautomatically updated.